If they have cleared it, the BMW garage may not be able to see what the code was. If it comes back on, I would deffo pop it to the BMW garage or an indy to have the code read. Alternatively, go to a normal garage, find out what the code it, leave the light on then pop to a main dealer.
I had my emissions light on a few weeks back, took it to an Indy who cleared it, spoke to BMW as it was under warranty and they said they couldnt do anything until the light came back on.
On another note, if it is "just" out of warranty, and if you do take the car to BMW for your servicing etc and its all up to date, and then take it to them with this fault, then thye may have fixed it out of goodwill or offered a reduction in cost to repair.
Out of interest - what was the code/fault it returned when RAC checked it?
!!